The present invention relates to a combiner and a method of forming a combiner.
A combiner is conventionally formed from a first glass prism having a surface arranged to carry a dielectric coating applied to the surface, such that the dielectric coating is arranged to reflect light from an image, and a second glass prism having a surface bonded to the surface of the first glass prism using optical cement. In this manner, when a user, typically a pilot of an aircraft, views a scene through the combiner, the image appears to be superimposed on the scene. It will be understood that an image generator, such as a cathode ray tube, flat panel display or another suitable image generating means, can generate an image containing information such as primary flight data relating to the aircraft, for example, navigation and guidance information or cues for the user to follow. Therefore, the image can be conveyed to the user so as to overlay a scene viewed by the user through the combiner such that the user does not need to look away from the scene, for example to read primary flight data from a head down display, and the image is provided at infinity such that the eye of the user does not need to be refocused in order to view the image.
However, if the combiner is to be used as part of or an attachment to headgear, such as a helmet or goggles, worn by the user then the weight of the combiner will increase the mass required to be supported by the head and particularly the neck of the user, thereby increasing user fatigue when using the headgear. Furthermore, the safety of the user can also be compromised, particularly during ejection from the aircraft. Accordingly, the headgear or a part thereof needs to be removed prior to ejection of the user from the aircraft.
The weight of the combiner can be reduced by replacing the first and second glass prisms with a pair of plastic prisms formed from one of a range of optical plastics. Such plastics have been used to form lenses in optical systems for many years where light weight, safety or cost effectiveness of a mass produced lens is desirable. However, these plastics are relatively unstable materials to polish or to manufacture to a high precision, for example by moulding, due to their physical properties. When such plastics are polished or moulded they deform under manufacturing stress which causes the finished surface of the prism to differ to that original intended. Furthermore, the surface of the plastic is soft making a smooth polished finish difficult to achieve.
Furthermore, perhaps more importantly, without special techniques, which are still under development, an optical coating applied to a plastic surface is of a much poorer quality than an optical coating applied to a glass surface. That is the optical coating tends to craze thereby hazing the user""s vision of a scene viewed through the combiner or the optical coating may peel from the plastic surface.
It is an object of the present invention to obviate or mitigate the disadvantages associated with the prior art.
According to a first aspect of the invention, a combiner comprises a glass reflector having a surface, a plastic prism having a surface arranged to adhere to the surface of the glass reflector, and a bonding agent arranged to adhere the surface of the glass reflector to the surface of the prism, wherein the bonding agent is arranged to allow the glass reflector and the plastic prism to expand and contract at different rates with respect to one another.
In this manner, the different rates of expansion and contraction, that is the difference in the expansion coefficient between the glass reflector and the plastic prism can be absorbed by the bonding agent used to adhere the glass reflector to the plastic prism during thermal cycling.
The surface of the glass reflector may be curved and the surface of the plastic prism may be arranged to complement the surface of the glass reflector. The surface of the glass reflector may also comprise a dielectric coating. In this manner, an optical coating can be disposed at the surface of the glass reflector thereby inhibiting crazing associated with disposing an optical coating on a surface of a plastic prism.
Preferably, the glass reflector may have a second surface arranged to adhere to a surface of a second plastic prism, and a bonding agent may be arranged to adhere the second surface of the glass reflector to the surface of the second plastic prism. The second surface of the glass reflector may also comprise a dielectric coating. The second surface of the glass reflector may be curved and the surface of the second plastic prism may be arranged to complement the second surface of the glass reflector.
Alternatively, the plastic prism may have a second surface arranged to adhere to a surface of a second glass reflector, and a bonding agent may be arranged to adhere the second surface of the plastic prism to the surface of the second glass reflector. The surface of the second glass reflector may also comprise a dielectric coating. The surface of the second glass reflector may be curved and the second surface of the plastic prism may be arranged to complement the surface of the second glass reflector.
The or each glass reflector may be a pellicle reflector. The bonding agent may be a flexible silicone adhesive.
Preferably, the combiner may be an eyepiece combiner arranged to form part of a helmet mounted display system. A combiner according to the invention is lighter than a conventional combiner formed from glass incorporate within headgear to be worn by a user, therefore the combiner reduces the loading of the headgear on the neck of the user and the user may eject from the aircraft while wearing the headgear incorporating the combiner.
According to a second aspect of the invention, as method of forming a combiner includes forming a surface on a glass reflector, forming a surface on a plastic prism and arranging the surface to adhere to the surface of the glass reflector, and bonding the surface of the glass reflector to the surface of the prism, such that the bonding agent is arranged to allow the glass reflector and the plastic prism to expand and contract at different rates with respect to one another.
The method may include priming the surface of the glass reflector and the surface of the plastic prism prior to bonding the surfaces to one another. The method may include coating the surface of the glass reflector with a dielectric coating prior to priming the surface of the glass reflector.
Preferably, the method may include forming a second surface on the glass reflector, forming a surface on a second plastic prism and bonding the second surface of the glass reflector to the surface of the second plastic prism. The method may include priming the second surface of the glass reflector and the surface of the second plastic prism prior to bonding the surfaces to one another. The method may include coating the second surface of the glass reflector with a dielectric coating prior to priming the second surface of the glass reflector.
Alternatively, the method may include forming a second surface on the plastic prism, forming a surface on a second glass reflector and bonding the second surface of the plastic prism to the surface of the second glass reflector. The method may include priming the second surface of the plastic prism and the surface of the second glass reflector prior to bonding the surfaces to one another. The method may include coating the surface of the second glass reflector with a dielectric coating prior to priming the surface of the second glass reflector.
The method may include forming the or each surfaces such that it is curved.